Recently, there have been major advances in the efficiency of photovoltaic (PV) devices. For example, using the basic GaAs material plus some replacement element like Indium, Spectrum Labs (now a division of Boeing) has reached a world record efficiency of ½ of the theoretical limit of infinite layers at 87% [2] with concentrator, and approximately 30% without concentrator using the triple junction structure. Such a PV device, however, is notoriously difficult to fabricate and form an integrated unit where the same current would flow over layer by layer without waste and with photons being collected in their designated regions. To improve on the silicon PV efficiency at 21-22% in order to reach the triple junction level at 30%, the difference in price per Watt could imply a price change of 104, therefore only the specialty application such as for space, for example, would engage the far more costly triple junction solar cells. Note that using n-Si wafers, MidAmerican SunPower could reach an efficiency of nearly 24% [10] at a much reduced price.
The recent announcement of using PbSe nanocrystal with the biological molecule pentacene to harvest the solar radiation with the triplet excitons from a group at Cambridge University [1] that has reached an efficiency of 95% is astonishing. Although the use of biological molecules is beyond this solicitation, it does involve compound semiconductors in nanocrystal, an area beyond the CQD's current investigation and we will watch its development to see if the triplet state energy could be collected in the singlet ground.
The efficiency of recent flexible colloidal PV cell with QD at 8-10% [2] seems to have been surpassed by the equally recent, flexible perovskite PV cells at twice the efficiency [11]. They do not, however reach the proposed CQD on the simple AlP/Si heterojunction solar cell that could have ˜40% efficiency with even lower unit fabrication cost and unit facility cost.
Reviewing some recent new developments, the triplet exciton approach using the biochemical molecule pentacene by the Cambridge group has claimed an astonishing quantum efficiency of 95% [1], which is higher than the theoretical limit of an infinite number of multijunction layers following exactly the solar radiation spectrum with ideal thermal management at 87% [2], and the IBM/Canadian Universities' PV approach of using a flexible substrate coated with black nano particles at 8% efficiency [3]. These new developments offer certain new perspectives and possibilities. There is a question, however, for the triplet exciton model, once the “device” is connected with electrodes to collect the photoelectrons, would the EF force the triplet electrons to return to the singlet ground state? Also for the flexible substrate coated with black nano particles, it seems that the system will be such a perfect heat sink for sunshine, that it could instead be more useful for thermal electric power generation, also using quantum dots, and deliver 30% power efficiency with appropriate insulation in order to reach the needed 600° C.
Using a solar constant of 1,366 Watts/m2 at AMO, each km2 of area facing the sun receives solar radiation energy equal to a mid-sized nuclear power plant. For space born vehicles with high launching cost, the most desired parameters for solar cells include low specific weight, high photovoltaic (PV) efficiency, high durability in the space environment and high tolerance to radiation, as well as a relatively low fabrication cost. Nearly all these demanding parameters can be met by heterojunction solar cells enhanced by charged quantum dots (CQD) to be manufactured at a low fabrication cost as described hereinafter.
At an anticipated 40% PV efficiency, and achieving it at low cost for realistic implementation can benefit not only space-borne vehicles and unmanned air vehicles (UAV), but also low cost terrestrial applications. In fact, compared to triple junction PV cells reaching the world record efficiency with concentrator and a space device at ˜30%, it is contemplated that the device described herein may be able to surpass that record efficiency without concentrator and achieve it at a cost reduction of many orders of magnitude. From space-borne units to UAV to low cost commercial roof-top installations, there is a large range of potential applications and the high throughput fabrication scheme is so inexpensive that it can dramatically reduce the capital requirements to accelerate the development of these applications.